Known from the state of the art are vibronic limit level switches, which monitor the limit-level of a liquid or a bulk good with the assistance of a mechanically oscillatable unit. The mechanically oscillatable unit, which is, for example, a rod in the case of bulk goods and a fork with two tines in the case of liquids, is excited such that it executes resonant oscillations. The oscillation excitement occurs, in such case, as a rule, by means of a piezoelectric or inductive transducer. If the degree of coverage of the oscillatable unit with the medium changes, whose limit-level is to be monitored, this influences the oscillation frequency of the oscillatable unit. By establishing a limit value and comparing the current oscillation frequency with the limit value, it can be detected whether the fill level of the medium exceeds or subceeds the limit-level. Modern vibronic limit level switches are able to determine the limit-level reliably even in the case of strongly accretion forming or foaming media. Such limit level switches are produced and sold by the applicant under the mark Liquiphant in a large number of variants.
Further known are limit level switches having at least one electrode and utilizing the capacitive or conductive principle. Another electrode or the container functions as counter electrode. The limit-level is measured capacitively via the capacitance of the capacitor formed of electrode and counter electrode varied by the medium acting as dielectric. In the case of conductive measuring, the conductive medium short circuits the electrode and the counter electrode.
The above-described limit level switches require electrical energy for measuring. The energy supply occurs, as a rule, via a 2-conductor connection with a voltage source. Via a third line, the limit level switch forwards the switch signal to a recording apparatus or to a control unit, for example, a programmable logic controller (PLC), for control of downstream field devices.
Making do without auxiliary energy and thereby with only two conductors are so called mechanical float switches. These are formed of a rod, which is introduced vertically into the measured liquid, and one or more magnetic floating bodies, which are freely movable in the vertical direction but guided by the rod. If the float reaches the fill level to be monitored, it closes an electrical contact and thereby connects an output and an input of a processing unit. Such mechanical float switches operate without auxiliary energy but are more wear susceptible than, for example, vibronic or capacitive, limit level switches.